Tuesday, August 31, 2010

crosswinds, stability, and trail from Bicycle Quarterly

Moving backwards through the four copies of Bicycle Quarterly I recently received, I came across a very interesting article in the Spring 2010 edition. It addressed a topic I'd been puzzled about before: the relationship between trail and cross-wind stability.

It's well established that trail contributes to bicycle stability. For example, this reference describes trail as a generally positive thing for stabiltiy: more trail = more stable. So if I'm riding in a cross-wind with deep-dish wheels, I should want a lot of trail, obviously.

Yet of my two bikes, a Ritchey Breakaway (somewhat slack 72.5° head tube angle) and my Fuji SL/1 (super-slack 71° head tube angle), the Ritchey seems to do better in cross-winds. What's up with that? Everything I'd read says trail is what makes bikes move in a straight line. That should apply to cross-winds as well as to other conditions, right?

Well, not quite.

What trail does is cause the bike to steer when it leans. This is normally a good thing for stability, since generally you want the bike to be vertical relative to gravity if it's going in a straight line: the bike starts to lean, the front wheel turns due to the trail, and the bike steers into the lean, moving the contact patches back under the center of mass and keeping the bike + rider upright. A bike with too much trail really, really "wants" to stay upright. If you push it without a rider, it will coast further before crashing than a bike with little trail, all other things equal.

But a cross-wind changes things. When a cross-wind hits a rider, the condition of static balance is for the rider to lean into the wind. With trail, the front wheel turns in response to the lean. This is not at all what's wanted: the rider therefore needs to turn the handlebars to fight the tendency of the bike to steer. Since "wheel flop" is generally greater with a bike with more trail (wheel flop = sin θ cos θ T, where θ is the head tube angle and T is the trail), and wheel flop determines how hard it is to straighten the handlebars when the bike is leaned over, a bike with more trail or a slacker head tube needs more force to keep the bike moving in a straight line. The Fuji has both more tail and a slacker head tube: a flop-double-whammy.

So while that slack head tube may be nice for carving lines through corners (some of that benefit lost due to the greater flop), it's not a good thing when there's a steady side-wind.

So my perceptions weren't so crazy after all: the Ritchey may well be, in theory as well as in practice, the better cross-wind frame.

4 comments:

Dan, do they say anything about the influence of deep section wheels? On the front, the larger surface area and the fact that the center of pressure is offset from the steering axis will tend to work in the opposite direction of the lean effect, no? (Think of it in terms of "countersteering"). My suspicion is that the combination of these 2 effects is why they commonly noted observation of TT bikes being MORE stable with a rear disk in sidewind conditions is true.

Yeah -- that's an important factor. In fact, this blog post doesn't really describe what BQ discusses, which is a steering moment induced by the force of wind on the wheel itself, as opposed to on the rider.

The issue is Bicycle Quarterly claims this steering moment is proportional to the trail (and also proportional to the wheel dish, approximately), and I've been trying to wrap my head around that one. I'll do my next blog post on that subject, likely.

I have been thinking about this myself recently. Maybe I need to build an adjustable steering geometry frame and do some testing. I would like to see the BQ article, I have never seen this publication here in Australia.